YELLOWTAIL FLOUNDER OFF NEW ENGLAND 



225 



appear to have been augmented and spread out on 

 cruise 3 (May 19 to 23), a pattern that continued 

 to cruises 5 (June 1 to 5) and 6 (June 5 to 8). 

 Beginning on cruise 6 and more noticeable on 

 cruise 7 (June 15 to 19) is the reduction in the 

 numbers of larvae found off New Jersey and Long 

 Island as compared with those found off southern 

 Massachusetts. 



The distribution of the large larvae (fig. 28) 

 was in most respects similar to that of the small, 

 the principal differences being smaller numbers 

 and the lesser variation in the catches of the large 

 larvae. It is as though the peaks occurring in 

 the distribution of the smaller larvae had had the 

 opportunity to disperse somewhat. 



The movement of one of the centers of distribu- 

 tion of the yellowtail is consistent with previous 

 estimates of drift and, incidentally, provides an 

 estimate of the duration of the small larval stage. 

 In his study of the mackerel, Sette (1943) was 

 able to identify and follow for a considerable 

 period certain peaks in the frequency distributions 

 of mackerel larvae, which he judged were pro- 

 duced by homologous groups that resulted from 

 fluctuations in spawning. The movement during 

 its passive phase of one of the most prominent of 

 these groups, which he called the S group, was 

 consistent witli the wind movement. First found 

 off Delaware Ray, this group moved about 60 

 miles south between cruises 1 and 2. The change 

 in distribution of the small yellowtail larvae from 

 that noted on cruise 1 and the northward move- 

 ment of the southern center of large larvae ob- 

 served on cruise 2 are in agreement with Sette's 

 observations of the mackerel. This drift, coupled 

 with the absence of small larvae at most of the sta- 

 tions where the southern center of large larvae 

 was found on cruise 2, suggests further that the 

 small yellowtail larvae progressed to the "large" 

 stage in the 5 or 6 days intervening between the 

 visits to the pertinent stations on cruises 1 and 2. 

 If this were so, probably the groups of small 

 larvae found on the later cruises had hatched from 

 successive spawnings. 



Further evidence of drift is suggested by the 

 fact that the center of small larvae that persisted 

 off Martha's Vineyard and/or Block Island from 

 cruise 2 through cruise 7 was not followed by any 

 special concentration of large larvae at these loca- 

 tions. We would expect a westerly or southwest- 

 erly drift to result from the prevailing coastal 



current — a conclusion strengthened also by Sette's 

 discovery of a southwesterly drift of the northern 

 center of larval mackerel off New Jersey during 

 cruises 1 to 3. Since no special concentration of 

 large larvae was found within a reasonable dis- 

 tance to the westward on cruise 3, these small 

 larvae must have drifted north or east beyond the 

 limits of the survey. 



Further analysis of the drift of these groups of 

 larvae appears fruitless because the yellowtail 

 larvae were obviously more widely distributed 

 than the mackerel which the cruises were designed 

 to cover. In none of the cruises was the eastern 

 limit of the yellowtail larvae included, and cruises 

 4, 6, and 9 (fig. 28) obviously did not cover the 

 southwestern limits of their distribution. Fur- 

 thermore, there was a considerable seaward spread 

 of the large larvae, for on cruises 4 and 6 large 

 larvae were found at every station that went to 

 the edge of the Continental Shelf. 



The depth distribution of yellowtail larvae 

 found on the station off Fire Island in 1929 (table 

 45) was evidently not always typical of the dis- 

 tributions in 1932. No data from a similar spe- 

 cial station are available for 1932, but at all of 

 the deeper stations two levels were sampled by 

 oblique, tows. These were designed to sample the 

 zone above the thermocline separately from the 

 zone below. At this time the thermocline was 

 usually about 20 meters down. On the average, 

 more larvae were taken above the thermocline 

 (appendix table E-3, p. 265), but at some stations 

 all of the larvae were found below it (e. g., sta- 

 tions II and III off Atlantic City), and there 

 were numerous instances of wide vertical dis- 

 tribution. No apparent relation existed between 

 this distribution of the larvae and any factors of 

 location, temperature, or time. 



Temperature relationships found on these 

 cruises (figs. 29 and 30) show the expected vernal 

 warming with variations due to weather. The sur- 

 face temperatures give evidence of a gradual sea- 

 sonal increase interrupted by an invasion of cold 

 water from the northeast at the time of cruise 2. 

 This was compensated for by a spurt in the warm- 

 ing between cruises 4 and 5 followed by a gradual 

 increase in water temperature through cruise 7. 

 We note that the larvae were found in numbers 

 when surface temperatures were as low as 8° C. 

 on cruise 2 and as high as 20° C. on cruise 7. 



